On a bit of a tangent, I came across information about openQA from openSUSE, which also compares images, but of desktop screenshots.

"openQA can test any operating system which is able to run in a virtual machine. By taking screenshots of the process and comparing these to reference images it determines if tests are passed or not. The OS-autoinst tool, part of openQA, controls the testing process by sending virtual keyboard and mouse events to the operating system being installed and run. It is able to respond to what is shown on the screen so it can handle a variety of issues while not having to stop the test."